This invention relates to a flowmeter for measuring a liquid metal flow at temperatures approximately up to 600.degree. C. in a pipe conduit having a nominal diameter of up to approximately 1,000 mm. The flowmeter includes a magnet for inducing an electric potential difference in the liquid metal flowing through the magnetic field and further has electrodes for measuring the potential difference.
Nuclear reactors which are cooled with liquid metal such as sodium can be operated only if the liquid metal flow is measured in all the principal coolant conduits and the requirement concerning redundancy and diversity of the flowmeters to be used is met in accordance with safety regulations. At each measuring location thus at least two redundant flowmeters are used which operate on unlike measuring principles.
An uncontrolled reduction in the coolant flow caused, for example, by the breakdown of a pump, leads to an insufficient heat removal and thus endangers the entire installation. Further, flow measurement is necessary for determining the reactor output, because only by means of a precise monitoring can the nuclear power installation be operated in an economical manner.
It is known to induce a potential difference in a flowing liquid metal and to measure the potential difference (which depends from the flow velocity) by means of electrodes and apply the signals to a data processor. For generating a magnetic field in the flowing liquid metal, at the outside of the conduit permanent magnets or coils are arranged whose dimensions and weight, particularly in case of large nominal pipe diameters, reach magnitudes that lead to structural and operational difficulties and render their use in a pool reactor unfeasible. The electrodes are welded to the outside of the coolant conduit. A redundant arrangement of several flowmeters of this known type which are independent from one another is not feasible because of the large weight and substantial spatial requirements. Flowmeters of these known types are discussed, for example, in C. G. Clayton, MODERN DEVELOPMENTS IN FLOW MEASUREMENT, pages 359 and 360, published by Peter Peregrinus Ltd., London 1972.
It is a further disadvantage of systems with permanent magnets that they are affected by aging and by temperature conditions and cannot be calibrated in situ.
It is further known to arrange in the flowing medium a drag body which generates in the medium a Karman vortex street as a free oscillation. The flow velocity is determined by measuring the vortex frequency. As sensors, for example, thermistors are used which are arranged at an upper face of the drag body or are situated in the inside thereof. Such a flowmeter is described, for example, by K. W. Bonfig, TECHNISCHE DURCHFLUSSMESSUNG, pages 146-148, published by Vulkan-Verlag, Essen, 1977. Measuring systems of this type are not adapted for measuring liquid metals because the thermistors cannot be used under temperature conditions prevailing in such an environment.
Further, German Laid-Open Application (Offenlegungsschrift) No. 2,632,042 discloses a probe-like permanent magnet flowmeter. In the probe there are arranged two disc-shaped permanent magnets at a predetermined distance from one another for inducing a first and a second voltage and electrodes for measuring the voltages. The flow velocity is determined by measuring the transit time of the velocity fluctuation. In case of large nominal conduit diameters, however, such a probe can respond only to a small part of the conduit cross section. Further, since the probe is submerged directly into the liquid metal and is to be arranged coaxially with respect to the conduit axis, it can be replaced only after entirely emptying the pipe conduit.